The temperature of a produced hydrocarbon containing stream at sub sea level is generally hot and may normally vary in temperatures in the range of 30-150° C. at the prevailing pressures. Depending on temperature, pressure and the components of the produced hydrocarbon, the produced hydrocarbon is usually obtained as a mixture of gases and liquid. The mixture may comprise multiple liquid phases, typically at least a hydrocarbon phase and at least an aqueous phase. In order to process or transport such a hot hydrocarbon liquid stream, it may be necessary to regulate the temperature of the stream, typically by cooling the stream to a specific temperature range of 0-60° C.
A known type of sub sea coolers consists of bundles of steel tubes in which the warm process stream flows. Seawater at approximately 4° C. freely flows across the outside of the tubes, directly cooling the process stream by heat dissipating through the steel tube walls to the surrounding seawater. The amount of cooling strongly depends on the sea current, as the heat transfer is increased multiple times if there is a strong sea current. In this way, the outlet temperature of the process stream is given by uncontrollable variables.
Hydrocarbon liquids may develop wax and hydrate depositions when cooled down below certain temperatures. A build up of wax and/or hydrate depositions inside tubes or process equipment will reduce the capacity of said parts and in worst case lead to blocking which stops productions and is both time consuming and costly to remove. Hence, uncontrolled cooling of hydrocarbon liquids at sub sea level is undesirable, as the outlet temperature of such coolers may result in undesirable process conditions, such as temperatures below the wax or hydrate formation temperature in the liquid. WO 2008/147219, (FMC Kongsberg Sub sea), describes a sub sea cooling unit having an inlet for a hot liquid and an outlet for cooled liquid, the cooling unit comprising a number of coiled tubes exposed to seawater, and means for generating a flow of seawater past the coils, where the means for generating the flow of seawater comprises a propeller and a pivotal actuator and wherein the cooler is enclosed in a duct.
WO 2010/002272, (Aker Sub sea AS), concerns a sub sea convection heat exchanger for cooling or heating a hydrocarbon-containing liquid in sub sea environment. The heat exchanger comprises a convection section with a liquid carrying pipe adapted for heat transfer between the carried hydrocarbon liquid on one side of the pipe wall and the surrounding water on the opposite side of the pipe wall. The convection section is enclosed by an enclosure with a seawater inlet and a seawater outlet and the heat exchanger is provided with means for controlled through-flow of the surrounding seawater from the seawater inlet to the seawater outlet.